The overall objective of this work is the development of novel chemotherapeutics and chemotherapeutic targets for the treatment of diseases of Mycobacterial origin including tuberculosis, leprosy, and M. avium-intracellulare (MAC) infections of AIDS patients. Pathogenic strains of mycobacteria uniformly modify their major mycolic acids by cyclopropanation while non-pathogenic strains do not. We have used this distinction to clone a gene from a pathogenic strain (M. tuberculosis) into a non-pathogenic strain (M. smegmatis) which cyclopropanates mycolic acids. By doing this we have created and characterized a novel mycolic acid which is composed of a hybrid between the normal M. smegmatis and M. tuberculosis major mycolic acids. Identification of the gene involved revealed that the encoded protein has high homology to the only known enzyme with a related function, the E. coli cyclopropane fatty acid synthase. In addition, sequencing the genomic DNA flanking this gene has revealed the presence of two new proteins which appear to be homologous to genes which would be involved in lipid biosynthesis. Thus we believe that we have cloned a cluster of biosynthetic genes which are related to mycolic acid biosynthesis. Each of these genes independently represents a potential drug target. We have also synthesized a novel class of inhibitors targeted towards cyclopropanated mycolic acid biosynthesis which are based on sulfur-substituted fatty acid derivatives and shown that these compounds selectively inhibit the growth of M. tuberculosis.